The present invention relates to seals in a gas turbine for supplementing the chordal hinge seals between turbine nozzles and a turbine nozzle support ring and particularly relates to supplementary seals for substantially minimizing or eliminating leakage losses past the chordal hinge seals.
In a gas turbine, hot gases of combustion flow from combustors through first-stage nozzles and buckets and through the nozzles and buckets of follow-on turbine stages. The first-stage nozzles typically include an annular array or assemblage of cast nozzle segments each containing one or more nozzle stator vanes per segment. Each first-stage nozzle segment also includes inner and outer band portions spaced radially from one another. Upon assembly of the nozzle segments, the stator vanes are circumferentially spaced from one another to form an annular array thereof between annular inner and outer bands. A nozzle retaining ring coupled to the outer band of the first-stage nozzles supports the first-stage nozzles in the gas flow path of the turbine. An annular nozzle support ring, preferably split at a horizontal midline, is engaged by the inner band and supports the first-stage nozzles against axial movement.
In an exemplary arrangement, eighteen cast segments are provided with two vanes per segment. The annular array of segments are sealed one to the other along adjoining circumferential edges by side seals. The side seals seal between a high pressure region radially inwardly of the inner band, i.e., compressor discharge air at high pressure, and the hot gases of combustion in the hot gas flow path which are at a lower pressure.
Chordal hinge seals are used to seal between the inner band of the first-stage nozzles and an axially facing surface of the nozzle support ring. Each chordal hinge seal includes an axial projection which extends linearly along a chordline of the inner band portion of each nozzle segment. Particularly, the chordal hinge seal extends along an inner rail of each segment and which rail lies radially inwardly of the inner band portion. The chordal hinge seal projection lies in sealing engagement with the axially opposite facing sealing surface of the nozzle support ring.
During operation and/or repair of the first-stage nozzle, it has been found that warpage can leave gaps between the chordal hinge seals and the sealing surface of the nozzle support ring. These gaps enable leakage past the chordal hinge seals from the high pressure area radially within the annular inner band into the hot gas flow path. That is, the chordal hinge seals are inadequate to prevent leakage flow as the chordal hinge seal projections lose contact with the sealing surface of the nozzle support ring. Consequently, there is a need for a supplemental seal at the interface of the first-stage nozzles and nozzle support ring to minimize or eliminate the leakage flow past the chordal hinge seals.
In accordance with a preferred embodiment of the present invention, there is provided a supplemental seal between the first-stage nozzles and the nozzle support ring which eliminates or minimizes leakage past the chordal hinge seals and which is readily and easily installed. In a preferred embodiment, the supplemental seal includes a flexible sheet metal seal secured to the inner rail and having a margin preloaded for sealing engagement against a first surface of the nozzle support ring. Preferably, the sheet metal seal is bent or folded in a generally L-shaped configuration with the two leg portions of the sheet metal seal extending along a side of the inner rail remote from the chordal hinge seal and along a radial inner face of the inner rail, respectively. The sheet metal seal extends the full chordal length of the inner rail. At the juncture of the leg portions, the sheet metal seal is relieved to provide a bead directed away from the corner edge of the inner rail. The margin of the sheet metal seal is preloaded and extends axially and then turns in a generally radially inward direction to seal against the first surface of the nozzle support ring.
To retain and clamp the sheet metal seal segment onto the inner rail, a complementary-shaped bracket overlies the seal and inhibits or prevents leakage flow between the sheet metal seal and the inner rail. The bracket is preferably either welded to the inner rail along its radial outer edge and along ends of the bracket and the inner rail or the bracket may be bolted to the inner rail. In either case, the bracket supports the seal, clamps the seal against the inner rail, provides the sealing support between the seal and the inner rail and maintains the preloaded margin of the sheet metal seal in sealing engagement against the first surface of the nozzle support ring.
In a preferred embodiment according to the present invention, there is provided a turbine comprising a turbine nozzle support ring having a generally axially facing first surface, a turbine nozzle segment having at least one stator vane and a radially inwardly extending inner rail having a second surface in axial opposition to the first surface, a seal extending along the inner rail including a sheet metal segment having a flexible margin preloaded to seal against the first surface and a support bracket extending along at least a portion of one side of the seal to support the sheet metal segment along the inner rail, the support bracket being secured to the inner rail to clamp the sheet metal segment against a wall portion of the inner rail and minimize or prevent leakage between the rail and the sheet metal segment.